feat(overview): show message dates

[keilogic]

Part of #1482.

Summary

• Expose the indexed message date on SearchIndexDocumentData from the existing Xapian date string value.
• Show that formatted date before each subject in the Overview sender highlights.
• Keep Overview rendering tolerant of messages without an indexed date, and add focused component coverage for both dated and undated messages.

Testing

• npm ci
• npx ng test --watch=false --browsers=FirefoxHeadless --include=src/app/start/sender-hilights.component.spec.ts (2 success)
• npx tsc -p src/tsconfig.app.json --noEmit
• npx tsc -p src/tsconfig.spec.json --noEmit
• npx eslint src/app/start/sender-hilights.component.ts src/app/start/sender-hilights.component.spec.ts src/app/xapian/searchservice.ts --quiet
• npm run lint -- --quiet
• git diff --check origin/master...HEAD
• git show --stat --oneline --check HEAD
• npm run policy (current commit OK; historical upstream commit-message warnings still printed)
• $env:SKIP_CHANGELOG='1'; node src/build/pre-build.js; npx ng build --configuration production --base-href=/app/ runbox7; node src/build/post-build.js (production build hash 6570d1a892a1ff90; existing Angular/Sass/CommonJS warnings)

AI Disclosure

I used OpenAI Codex/GPT-5 in a local development workspace to inspect the relevant code, implement this change, and run the validation commands listed above. I reviewed the resulting diff and validation output before submitting.